1. Technical Field of the Invention
This invention relates to a scroll compressor.
2. Description of the Related Art
Generally, a scroll compressor includes a scroll fixed on a housing and a movable scroll arranged in opposed relation to the fixed scroll and adapted to revolve with respect to the fixed scroll on a rotary shaft, so that a fluid is compressed by the fixed scroll and the movable scroll. The movable scroll is subjected to the force in thrust direction by the pressure difference between the back surface of the movable scroll and the compressed fluid. This force in thrust direction is supported by a thrust bearing.
Since the movable scroll orbits, the sliding speed is lower in the case where the thrust bearing is used with the scroll compressor than in the case where the thrust bearing is used with a rotating device. As a result, it is difficult for the lubricating oil to form an oil film on the sliding surfaces, often resulting in seizure.
In a compressor included in a refrigeration cycle using a carbon dioxide refrigerant, the pressure of the compressed refrigerant is high enough to cause a large amount of force in thrust direction, so that forming an oil film on the sliding surfaces of the thrust bearing becomes a crucial problem.
Also, the scroll compressor has a large pressure-receiving area, which contributes to the problem of forming the oil film on the sliding surfaces, as described above.
A scroll compressor using carbon dioxide as a refrigerant for automotive vehicles is available, which has a thrust bearing with a pair of sliding surfaces formed of planar flat plates. In the case where an excessively large load is placed on the sliding surfaces, the oil film between the sliding surfaces becomes inconsistent and resulting in seizure.
Further, in the scroll compressor, the movable scroll orbits, compressing the fluid in the compression chamber, and therefore moves in a radial direction. Thus, the rotation moment around an axis perpendicular to the revolving axis acts (tilt moment) on the movable scroll and causes an offset thrust load, which results in more severe loading conditions.
Conventional scroll compressors incorporating various designs of the sliding surfaces have been proposed.
JP3426720B discloses a technique in which a multiplicity of minuscule oil pools having minuscule holes are formed on the sliding surface of the thrust bearing arranged on the back surface of the movable scroll and the lubricating oil is held by adsorption on the wall surface of the minuscule oil pools.
According to the technique described in JP3426720B, however, the lubricating oil is held by the wall surface of the holes of minuscule oil pools, and the minuscule oil pools are formed independently of each other. Thus, since the diameter and depth of each minuscule oil pool cannot be increased, the amount of the lubricating oil that can be held is limited. In the case where the compressor is operated with the lubricating oil failing to be supplied in minuscule oil pools for a long period of time, the lack of oil supply generates negative pressure on the sliding surfaces, resulting in that the sliding surfaces stick to each other, thereby resulting in a possible seizure.
The thrust bearing described in JP3426720B is arranged over the whole back surface of the movable scroll. With the movement of the thrust bearing due to the orbiting motion of the movable scroll, a part of the minuscule oil pools is displaced out of the mating side, resulting in that the area formed by the lubricating oil film is reduced.
Also, in a scroll compressor including a thrust bearing having a sliding surface of a movable scroll and a fixed sliding surface, a back pressure mechanism for applying pressure to the back of the shaft of the movable scroll to reduce the load imposed on the sliding surfaces has been proposed. This mechanism, however, requires a complicated control operation, and increases cost.
JP8-319959A discloses a scroll compressor with a plurality of taper land bearing mechanisms formed on the thrust bearing surface supporting the movable scroll, wherein the taper land bearing mechanism is formed with a multiplicity of tapered portions inclined in the direction of revolution and a multiplicity of circular land portions of predetermined height.
However, in the scroll compressor described in JP8-319959A, which is intended to form an oil film by a wedge effect on the sliding surfaces of the thrust bearing, the dimensions of the tapered portions and the land portions are not specified, and fluid lubrication is not necessarily obtained while in operation. Depending on operating conditions, a mixed lubrication or boundary lubrication may occur, often damaging the sliding surfaces of the thrust bearing due to friction and wear.
JP8-319959A lacks a description of the material and heat treatment of the bearing portion in order to secure wear resistance in the boundary or mixed lubrication region, which may occur when starting the compressor or “liquid back” (which is defined as a phenomenon in which a liquid-phase refrigerant is introduced into the scroll compressor together with a gas-phase refrigerant).
FIG. 26 is a plan view showing the sliding surface 134a of the movable scroll 132 of the conventional scroll compressor. This movable scroll 132 has a boss 135 at the central portion thereof coupled to an eccentric shaft (not shown), a sliding surface 134a on the outer periphery (hatched) and an inner peripheral non-contact surface 134b lower in level than the sliding surface 134a. An anti-rotation mechanism comprised of an Oldham ring (not shown) is often arranged on the back of the movable scroll due to the limited body size of the compressor. Therefore, a groove for establishing the anti-rotation mechanism is required to be arranged on the sliding surface of the housing or the movable scroll. In the prior art shown in FIG. 26, the key slots 142 are oblong and arranged in such a manner as to intrude into the area of the sliding surface 134a. Thus, the inner peripheral edge of the sliding surface 134a is segmented, and portions designated by a are formed at the corners adjacent to the key slots 142, and wear or seizure may occur at the parts a.
In the case where the tilt moment and the thrust load described above act on the movable scroll 132 in revolution, a precession is generated and the thrust load with the point of generation of the maximum thrust load moved along the circumferential direction acts on the sliding surface 134a while at the same time forming a high contact pressure portion along the inner peripheral edge of the sliding surface, resulting in that contact pressure rises at the parts a on the inner peripheral edge of the sliding surface with the pressure-receiving area reduced by the key slots 142, and also the parts a are disadvantageously adjacent to the key slots 142 deeper than the non-contact surface 134b for the oil supply operation.